1. Field of the Invention
The present invention relates to equalizing devices, equalizing methods, and signal transmitting devices.
2. Description of the Related Art
Because of recent speed-up of communication, transmission losses of communication media such as metal cables are increasing. Normally, signals transmitted through a communication medium are input to a linear equalizer having waveform equalizing function. In a communication medium such as a metal cable, losses of a conductor due to a skin effect are dominant at low frequencies, and, on the other hand, dielectric losses are dominant at high frequencies. In other words, when signals are transmitted through a metal conductor, the signals undergo two types of losses, i.e., a conductor loss and a dielectric loss. The signals are affected by both of them at low frequencies. However, as the frequency is increased, the influence of the conductor loss reduces, and the influence of the dielectric loss increases. JP 2014-050104 A describes a system having a low-frequency equalizer, which compensates for the loss at low frequencies, and a high-frequency equalizer, which compensates for the loss at high frequencies.
Incidentally, a linear equalizer has to expand a band to the band that is equal to or higher than a Nyquist frequency (data rate: the frequency which is half of a symbol rate). In this case, if a high-frequency gain of a high-frequency equalizer is increased in order to equalize a high-frequency side, insufficient equalization tends to occur near the frequency (intermediate frequency) at which the inclination of frequency characteristics of a communication medium changes even if a low-frequency equalizer is added like JP 2014-050104 A. Therefore, the inclination of the after-equalization frequency characteristics does not become constant, and the bit error rate thereof is deteriorated.
When the frequency characteristics of an output with respect to an input of a signal are to be calculated, a transfer function for calculating the ratio of an input and an output which have undergone Laplace transform is used. The Laplace transform is a method to convert a signal in the time domain to that in the frequency domain. When a signal x(t) in the time domain is to be converted to the frequency domain, the conversion is carried out by Equation (1). Herein, s=jω (“j” is an imaginary number, and “ω” is an angular frequency) is satisfied. The transfer function is expressed by “s”, a pole (pl), and a zero point (zk), and the absolute value of the transfer function is a gain of the output with respect to the input. Therefore, if the zero point is provided, the gain increases as the frequency increases.X(s)=∫t=0∞x(t)·e−stdt  (1)